Turbine blades used in gas turbine engines can suffer corrosion which limits the life of the component in the harsh conditions in which the turbine blade operates. Sulphidation is one form of corrosion to which the blade is susceptible and which can require early removal of the blade from the engine.
Turbine blades are typically nickel, cobalt or iron based superalloys and conventional environmental protection coatings include silicide modified aluminide coatings or chromised coatings for lower temperature Type 2 corrosion resistance.
In one coating described in U.S. Pat. No. 6,565,931 a diffused platinum coating is applied to turbine blade aerofoils and root shanks via an electroplating process and heat treated at 1100° C. to 1150° C. Applicable to Nickel based super-alloys, the coating is used as either an aerofoil bond coat or for sulphidation protection on the shanks of turbine blade roots. The platinum coating has three layers with the outer layer comprising both a gamma phase and a gamma prime phase. The outer layer of the metallic article may comprise a controlled of chromium in quantities of about 3% with an increasing level of 3-6% and 6.5% for the intermediate and inner zone respectively. At these levels of chromium it has been found that the outer layer remains as a two phase layer.
Service inspections and laboratory testing have shown that the coatings whilst providing good corrosion protection are unable to provide sufficient corrosion protection for the modern large civil engines that have entered service over the last five years. This has led to blade lives having to be limited by sulphidation cracking that occurs once the coatings have been breached.
The diffused platinum coating that is limited by sulphidation cracking, has been shown to only provide moderate corrosion resistance under low salt fluxes. Further understanding of the coatings structure has shown the two phase gamma-gamma prime coating to be susceptible to attack down the phase boundaries within the coating. This is believed to be responsible for the variability in corrosion protection observed depending on the specific environmental conditions for each engine examined.
Another known process for increasing corrosion resistance is known as chromising, which is a process that increases the near surface level of chromium. For some of the older Ni based alloys this has been shown to offer an improvement in corrosion resistance under low to moderate corrosive environments. More recent turbine blade alloys, suitable for use in the majority of large civil engines including, have been shown to be incompatible with chromising, resulting in no improvement in corrosion protection. This was due to Tantalum, acting as an alloying element, which was found to concentrate in the diffusion front after chromising causing a decrease in expected life. Unexpectedly, the tantalum caused an increase in the rate of degradation due to corrosion.
Some corrosion resistant alloys include high quantities of aluminium that form a beta phase alloy. Beta phase alloys are generally brittle and may require frequent replacement and repair during service of the component
It is an object of the present invention that seeks to provide an improved environmental coating.